1. Field of the Invention
The present invention relates to a catheter, and in particular relates to a catheter used in medical surgeries carried inside blood vessels, selective injection of carcinostatic agents or angiography or the like.
2. Description of the Background Art
In recent years, it has become possible to replace certain medical surgical operations with medical surgeries that are carried directly inside blood vessels by inserting catheters into the blood vessels percutaneously to perform medical treatments to lesion parts of the blood vessels. In such inner-vessel medical surgeries, a catheter must be selectively inserted into a predetermined position in the narrow blood vessels which are meandered in complex and have many branches.
For example, when carrying out an inner-vessel medical surgery procedure so called as embolectomy which is to be performed to an aneurysm or arteriovenous tumor which sometimes appears in a blood vessel within the brain or the like, the tip of a small diameter catheter is selectively passed through a blood vessel of the brain until it reaches the lesion part or the vicinity thereof. Then, a liquid-type thrombus agent such as cyanoacrylate or solution of ethylene-vinylalcohol copolymer using dimethyl-sulfoxide as a solvent, a granulated-type thrombus agent such as polyvinylalcohol, or a thrombus member such as a tiny coil is brought to the tip of the catheter through a lumen thereof and then it is applied to or put on the lesion part from an opening formed on the tip or the lumen. When carrying out such procedures which involve the injection or application of a liquid-type thrombus agent, a medicine or a contrast agent into a narrow blood vessel, a narrow-diameter catheter must be selected and used in accordance with the purpose of the procedure.
Accordingly, these small-diameter catheters must have operability by which the catheter can be passed through a complex network of narrow blood vessels quickly and reliably in addition to chemical and biochemical safeness which are always required in commercially available catheters. Furthermore, in order to carry out injection of the thrombus agents as described above, the catheter is also required to have a sufficient chemical resistance, and in particular to have a sufficient solvent resistance which can prevent alternation from being caused in the catheter when the catheter contacts such solvents as dimethyl-sulfoxide (DMSO) or the like.
In more detail, the operability of the catheter described above includes the following characteristics or abilities. Namely, a catheter is required to have sufficient pushability and torque transmission ability. The pushability means the characteristics of the catheter that can reliably transmit a pushing force given by an operator at the base end of the catheter to the distal end thereof. The torque transmission ability means the characteristics that can reliably transmit rotational force applied to the base end of the catheter to the distal end thereof. Further, it is also required for a catheter to have follow-up characteristics and kink resistance characteristics. The follow-up characteristics mean the ability by which the catheter can advance smoothly within meandered blood vessel along a guide wire which has been in advance inserted into the blood vessel without causing injury to the blood vessel walls. The kink resistance characteristics mean the ability of the catheter that can prevent it from being bent by bent portions or curved portions of the blood vessel even after the catheter has reached the object position and the guide wire has been removed. Moreover, in order to effectively exhibit these characteristics, the outer surface of the catheter must possess lubrication characteristics.
These characteristics required to the catheter becomes difficult to obtain as the diameter of the catheter is reduced. As for the prior art for giving the pushability and the follow-up characteristics, there has been developed and proposed a catheter having a main body portion at the proximal side thereof which is formed into a double or two layer structure. The double layered structure is formed from a relatively rigid inner tube and a relatively flexible outer tube which covers the outer surface of the inner tube and has a portion that protrudes over the tip end of the inner tube.
One such example of the prior art catheter is disclosed in National Publication of the translation version of Utility Model No. 60-500013. The catheter is provided with an inner tube layer made of polyamide and an outer layer made of urethane, and the tip portion of the inner tube is formed into a tapered shape in such a manner that the inner diameter thereof is gradually increased. However, such a catheter has less solvent resistance since the outer tube layer is made of urethane, and this makes the catheter unsuitable for embolectomy use. Furthermore, in the prior art catheter, since there is a sudden change in rigidity at the boundary section between the portion comprised of the two tube layers of urethane and polyamide and the portion comprised of the single tube layer of urethane, the catheter is likely to be bent at that portion during operation. Namely, the catheter is likely to be bent or damaged due to the concentration of force at the boundary section between the two tube layers and the single tube layer. In particular, at the section in which only the single tube layer of urethane is arranged, stress is likely to be concentrated to the boundary section between the above two portions, because the single tube layer has fixed inner and outer diameters and has a uniform rigidity even at the tip portion thereof, which results in occurrence of kinks, Further, such structure makes it difficult for the catheter to follow the branches and bent portions within a blood vessel. That is to say, the prior art catheter has less follow-up characteristics.
Another example is shown in U.S. Pat. No. 4,636,346. The U.S. Patent discloses a guiding catheter which is comprised of a main section having a triple or three layer structure and a tip section having a double layer structure that extends from the tip of the main section. In this construction, the main portion has an intermediate layer having a relatively high rigidity provided between an inner layer and an outer layer. However, as the distal portion of the catheter is not provided with this intermediate layer, its torque transmission ability and kink resistance characteristics are not sufficient.
Yet another example is shown in Japanese Laid-Open Patent Publication No. 57-173065. This Laid-open publication discloses a catheter having a helically wound wire which is provided inside the catheter. This catheter is composed of a tube which extends along roughly the entire length of the catheter and a helically wound wire which terminates at the middle of the tube. However, in this catheter, since the helically wound wire is directly exposed inside the inner cavity of the catheter (lumen), there is the possibility that the lumen is clogged by a thrombus agent or a thrombus member. Further, there is also the possibility that a guide wire is caught by the helically wound wire when the guide wire is inserted. For these problems, it is required for an operator to operate the catheter carefully.
A further example is shown in U.S. Pat. No. 5,178,158. The U.S. Patent discloses a catheter having a single coil layer which is provided inside an intermediate layer made of polyimide and an outer layer made of polytetrafluoroethylene, in which the distal portion of the catheter is constituted so as to be more flexible than the proximal portion by making the pitch of the coil in the distal portion greater than the pitch of the coil in the proximal portion. However, in this catheter, since the single coil layer is formed of a coil wound in only one direction, the catheter has insufficient torque transmission and kink resistance characteristics. In particular, with regard to the torque transmission characteristics of the catheter, it depends on the rotational direction of the catheter. Namely, the torque transmission characteristics of the catheter are relatively good when the catheter is rotated in the direction that tightens the coil, but the torque transmission characteristics of the catheter are relatively poor when the catheter is rotated in the opposite direction, that is, the direction that loosens the coil. Accordingly, the catheter has limited torque transmission characteristics in one direction of rotation, and this results in poor operability for the catheter. Moreover, in the same manner as was explained in the preceding paragraph for the catheter disclosed in Japanese Laid-open Patent Publication No. 57-173065, because the coil is directly exposed inside the lumen, there is the possibility that the lumen is clogged by a thrombus agent and there is also the possibility that a guide wire is caught by the helically wound wire when the guide wire is inserted. Further, the sliding resistance of the guide wire is unavoidably increased. For these reasons, the operability of the catheter is not so good.